Saturn's icy satellites and rings investigated by Cassini - VIMS. III. Radial compositional variability

In the last few years Cassini-VIMS, the Visible and Infared Mapping Spectrometer, returned to us a comprehensive view of the Saturn's icy satellites and rings. After having analyzed the satellites' spectral properties (Filacchione et al. (2007a)) and their distribution across the satellites' hemispheres (Filacchione et al. (2010)), we proceed in this paper to investigate the radial variability of icy satellites (principal and minor) and main rings average spectral properties. This analysis is done by using 2,264 disk-integrated observations of the satellites and a 12x700 pixels-wide rings radial mosaic acquired with a spatial resolution of about 125 km/pixel. The comparative analysis of these data allows us to retrieve the amount of both water ice and red contaminant materials distributed across Saturn's system and the typical surface regolith grain sizes. These measurements highlight very striking differences in the population here analyzed, which vary from the almost uncontaminated and water ice-rich surfaces of Enceladus and Calypso to the metal/organic-rich and red surfaces of Iapetus' leading hemisphere and Phoebe. Rings spectra appear more red than the icy satellites in the visible range but show more intense 1.5-2.0 micron band depths. The correlations among spectral slopes, band depths, visual albedo and phase permit us to cluster the saturnian population in different spectral classes which are detected not only among the principal satellites and rings but among co-orbital minor moons as well. Finally, we have applied Hapke's theory to retrieve the best spectral fits to Saturn's inner regular satellites using the same methodology applied previously for Rhea data discussed in Ciarniello et al. (2011).Comment: 44 pages, 27 figures, 7 tables. Submitted to Icaru

Surface ages of mid-size Saturnian satellites

The observations of the surfaces of the mid sized Saturnian satellites made by Cassini Huygens mission have shown a variety of features that allows study of the processes that took place and are taking place on those worlds. Research of the Saturnian satellite surfaces has clear implications for Saturn history and surroundings. In a recent paper, the production of craters on the mid sized Saturnian satellites by Centaur objects was calculated considering the current Solar System. We have compared our results with crater counts from Cassini images and we have noted that the number of observed small craters is less than our calculated number. In this paper we estimate the age of the surface for each observed terrain on each mid sized satellite of Saturn. We have noticed that since there are less observed small craters than calculated (except on Iapetus), this results in younger ages. This could be the result of efficient endogenous or exogenous process(es) for erasing small craters and or crater saturation at those sizes. The size limit from which the observed number of smaller craters is less than the calculated is different for each satellite, possibly indicating processes that are unique to each, but other potential common explanations would be crater saturation and or deposition of E ring particles. These processes are also suggested by the findings that the smaller craters are being preferentially removed, and the erasure process is gradual. On Enceladus, only mid and high latitude plains have remnants of old terrains; the other regions could be young; the regions near the South Polar Terrain could be as young as 50 Myr old. On the contrary for Iapetus, all the surface is old and it notably registers a primordial source of craters. As the crater size is decreased, it would be perceived to approach saturation until D less than 2 km craters, where saturation is complete.Comment: Accepted for publication in Icarus, 40 pages, 11 figure

Magnetospheric considerations for solar system ice state

The current lattice configuration of the water ice on the surfaces of the inner satellites of Jupiter and Saturn is likely shaped by many factors. But laboratory experiments have found that energetic proton irradiation can cause a transition in the structure of pure water ice from crystalline to amorphous. It is not known to what extent this process is competitive with other processes in solar system contexts. For example, surface regions that are rich in water ice may be too warm for this effect to be important, even if the energetic proton bombardment rate is very high. In this paper, we make predictions, based on particle flux levels and other considerations, about where in the magnetospheres of Jupiter and Saturn the ∼MeV proton irradiation mechanism should be most relevant. Our results support the conclusions of Hansen and McCord (2004), who related relative level of radiation on the three outer Galilean satellites to the amorphous ice content within the top 1 mm of surface. We argue here that if magnetospheric effects are considered more carefully, the correlation is even more compelling. Crystalline ice is by far the dominant ice state detected on the inner Saturnian satellites and, as we show here, the flux of bombarding energetic protons onto these bodies is much smaller than at the inner Jovian satellites. Therefore, the ice on the Saturnian satellites also corroborates the correlation

Far-Ultraviolet Spectroscopy of Saturn's Moons Rhea and Dione

This thesis presents the first analysis of spatially resolved, far-UV reflectance spectra of Saturn’s icy satellites Rhea and Dione collected by the Cassini Ultraviolet Imaging Spectrograph (UVIS) during targeted flybys. The objective of this geochemical survey of Rhea’s and Dione’s leading and trailing hemispheres is to identify and explain the broad absorption feature centred near 184 nanometres. The secondary objective is to determine the presence of any minor impurities within the ice layers of these satellites by characterizing the location in wavelength space and shape of the 165-nm absorption edge due to pure water ice. To determine the precise position of the mid-point of the absorption edge, derivative spectroscopy is applied to the Cassini UVIS spectra. The presence of any non-water-ice molecules within the ice matrix and/or changes in the structure of the ice grains should cause a shift from the nominal position of the 165-nm absorption edge, or alter the slope of the absorption edge. The 165-nm absorption edge and 184-nm absorption feature have been observed in disk-integrated spectra of Rhea and other icy satellites in the Saturnian system. However, as of to date, there has been no satisfactory explanation for the 184 nm absorption feature. Cassini UVIS reflectance spectra are compared with modelled spectra derived using far-UV spectra of thin-ice specimens collected in laboratory experiments and Hapke theory. The results of the modelling show that the UVIS observations can be modelled by two molecules: simple chloromethanes beneath a layer of water ice or hydrazine monohydrate mixed in solid phase with water ice. A detailed analysis based on the astrochemistry and geomorphology of Rhea and Dione show that chloromethane molecules on their surfaces is difficult to explain, since their presence would require active subsurface oceans and an extensive network of fractures through their thick ice shells. On the other hand, hydrazine monohydrate is easier to explain since it can be produced via irradiation of ammonia ice by energetic particles originating from Saturn’s magnetosphere environment. It is also shown in the research that hydrazine can also be produced on Saturn’s largest moon Titan by irradiation of ammonia ice, or by chemical reactions involving water-ice and solid ammonia ice. Several possible alternative scenarios based on an analysis of both sinks and sources of chloromethane molecules and hydrazine are presented in this thesis

Saturn satellites as seen by Cassini Mission

In this paper we will summarize some of the most important results of the Cassini mission concerning the satellites of Saturn. Given the long duration of the mission, the complexity of the payload onboard the Cassini Orbiter and the amount of data gathered on the satellites of Saturn, it would be impossible to describe all the new discoveries made, therefore we will describe only some selected, paramount examples showing how Cassini's data confirmed and extended ground-based observations. In particular we will describe the achievements obtained for the satellites Phoebe, Enceladus and Titan. We will also put these examples in the perspective of the overall evolution of the system, stressing out why the selected satellites are representative of the overall evolution of the Saturn system.Comment: 34 pages, 10 figures, to appear on the special issue of Earth, Moon and Planets for the Elba worksho

Enceladus and the Icy Moons of Saturn : July 26-29, 2016, Boulder, Colorado

Topics include (but are not limited to) geology, geophysics, geochemistry and mineralogy, active jets and their properties and origins, formation and evolution, astrobiology, and future investigations of these bodies. Comparative planetology is encouraged. A major focus will be the new results from the 2015 Cassini encounters with Enceladus and the other icy moons. Contributions from Cassini, Voyager, and groundbased studies are welcomeUniversities Space Research Association (USRA), Lunar and Planetary Institute (LPI)Local Organizing Committee, Larry Esposito, University of Colorado Carly Howett, Southwest Research Institute Laura Bloom, University of Colorado ; Science Organizing Committee Paul Schenk, Lunar and Planetary Institute [and 12 others

Origin and Evolution of Saturn's Ring System

The origin and long-term evolution of Saturn's rings is still an unsolved problem in modern planetary science. In this chapter we review the current state of our knowledge on this long-standing question for the main rings (A, Cassini Division, B, C), the F Ring, and the diffuse rings (E and G). During the Voyager era, models of evolutionary processes affecting the rings on long time scales (erosion, viscous spreading, accretion, ballistic transport, etc.) had suggested that Saturn's rings are not older than 100 My. In addition, Saturn's large system of diffuse rings has been thought to be the result of material loss from one or more of Saturn's satellites. In the Cassini era, high spatial and spectral resolution data have allowed progress to be made on some of these questions. Discoveries such as the ''propellers'' in the A ring, the shape of ring-embedded moonlets, the clumps in the F Ring, and Enceladus' plume provide new constraints on evolutionary processes in Saturn's rings. At the same time, advances in numerical simulations over the last 20 years have opened the way to realistic models of the rings's fine scale structure, and progress in our understanding of the formation of the Solar System provides a better-defined historical context in which to understand ring formation. All these elements have important implications for the origin and long-term evolution of Saturn's rings. They strengthen the idea that Saturn's rings are very dynamical and rapidly evolving, while new arguments suggest that the rings could be older than previously believed, provided that they are regularly renewed. Key evolutionary processes, timescales and possible scenarios for the rings's origin are reviewed in the light of tComment: Chapter 17 of the book ''Saturn After Cassini-Huygens'' Saturn from Cassini-Huygens, Dougherty, M.K.; Esposito, L.W.; Krimigis, S.M. (Ed.) (2009) 537-57

Surface ages of mid-size Saturnian satellites

The observations of the surfaces of the mid-sized saturnian satellites made by Cassini–Huygens mission have shown a variety of features that allows study of the processes that took place and are taking place on those worlds. Research of the saturnian satellite surfaces has clear implications not only for Saturn’s history and Saturn’s surroundings, but also for the Solar System. Crater counting from high definition images is very important and could serve for the determination of the age of the surfaces. In a recent paper, we have calculated the production of craters on the mid-sized saturnian satellites by Centaur objects considering the current configuration of the Solar System. Also, we have compared our results with crater counts from Cassini images by other authors and we have noted that the number of observed small craters is less than our calculated theoretical number. In this paper we estimate the age of the surface for each observed terrain on each mid-sized satellite of Saturn. All the surfaces analyzed appear to be old with the exception of Enceladus. However, we have noticed that since there are less observed small craters than calculated (except on Iapetus), this results in younger ages than expected. This could be the result of efficient endogenous or exogenous process(es) for erasing small craters and/or crater saturation at those sizes. The size limit from which the observed number of smaller craters is less than the calculated is different for each satellite, possibly indicating processes that are unique to each, but other potential common explanations for this paucity of small craters would be crater saturation and/or deposition of E-ring particles. These processes are also suggested by the findings that the smaller craters are being preferentially removed, and the erasure process is gradual. On Enceladus, only mid and high latitude plains have remnants of old terrains; the other regions could be young. In particular, the regions near the South Polar Terrain could be as young as 50 Myr old. On the contrary for Iapetus, all the surface is old and it notably registers a primordial source of craters. As the crater size is decreased, it would be perceived to approach saturation until DK 2 km-craters, where saturation is complete.Instituto de Astrofísica de La Plat

Surface ages of mid-size Saturnian satellites

The observations of the surfaces of the mid-sized saturnian satellites made by Cassini–Huygens mission have shown a variety of features that allows study of the processes that took place and are taking place on those worlds. Research of the saturnian satellite surfaces has clear implications not only for Saturn’s history and Saturn’s surroundings, but also for the Solar System. Crater counting from high definition images is very important and could serve for the determination of the age of the surfaces. In a recent paper, we have calculated the production of craters on the mid-sized saturnian satellites by Centaur objects considering the current configuration of the Solar System. Also, we have compared our results with crater counts from Cassini images by other authors and we have noted that the number of observed small craters is less than our calculated theoretical number. In this paper we estimate the age of the surface for each observed terrain on each mid-sized satellite of Saturn. All the surfaces analyzed appear to be old with the exception of Enceladus. However, we have noticed that since there are less observed small craters than calculated (except on Iapetus), this results in younger ages than expected. This could be the result of efficient endogenous or exogenous process(es) for erasing small craters and/or crater saturation at those sizes. The size limit from which the observed number of smaller craters is less than the calculated is different for each satellite, possibly indicating processes that are unique to each, but other potential common explanations for this paucity of small craters would be crater saturation and/or deposition of E-ring particles. These processes are also suggested by the findings that the smaller craters are being preferentially removed, and the erasure process is gradual. On Enceladus, only mid and high latitude plains have remnants of old terrains; the other regions could be young. In particular, the regions near the South Polar Terrain could be as young as 50 Myr old. On the contrary for Iapetus, all the surface is old and it notably registers a primordial source of craters. As the crater size is decreased, it would be perceived to approach saturation until DK 2 km-craters, where saturation is complete.Instituto de Astrofísica de La Plat